The present invention relates to a novel structure for a solid state electronic display device. Such devices are utilized in the output displays for digital clocks, watches, calculators, small computers, copiers, ovens, volt meters, and a wide variety of other devices. Such devices frequently must be visible to the human eye at a distance and it is desirable to make the display of sizes larger than a half inch. In displays larger than one half inch it is necessary to employ more than one light emitting device per segment to achieve uniform illumination along the length of the segment. (A numerical display would typically have seven segments.)
Previously, when more than one active element, such as a light emitting diode, (L.E.D.) was used per segment it was common to place each L.E.D. in its own recess formed in an optical shroud or frame. The optical shroud was typically made of a black or other dark material to prevent light coupling between segments and to improve contrast ratio. The shroud was formed to have angled surfaces to reflect light and the segment wall surfaces of the shroud were made highly reflective. The front faces of the shroud were silvered or aluminized. The L.E.D. was bonded to a drive circuit by way of a printed circuit board or a ceramic substrate and a shroud was placed over the L.E.D. and the circuit board. An epoxy was placed on the L.E.D.'s and in the openings of the shroud and cured; thus bonding the circuit board and the shroud together. The epoxy had a refractive index approaching that of the L.E.D. and in between that of the L.E.D. and air.
Because each light source has its own recess it was necessary for the best visual result to center the light source in the recess. This centering made it more difficult to get equal brightness into the corners of the display and resulted in a hot spot at the segment center. Because the human eye cosmetically rejects the appearance of digits having reduced intensity at their segments ends than those having more brightly illuminated corners than their centers. This effect is more troublesome when two or more segments intersect. It is cosmetically more pleasing to view digits whose corners are equal to or are more brighter than the intensity at the center of the segments.
The curing of the epoxy during manufacture and the subsequent treatment of the assembly after curing may result in failures in the whole package. Because the epoxy enclosed and sealed the whole assembly it was not economically feasible to repair any assembly found defective after final testing. Therefore the entire assembly had to be discarded greatly raising the cost of the manufacturing of these assemblies.
The closest patent art which is known to the inventors are: Katsumura, U.S. Pat. No. 3,886,581, issued, May 27, 1975; Usui, U.S. Pat. No. 3,869,637, issued Mar. 4, 1975; and Groves, U.S. Pat. No. 3,889,147, issued June 10, 1975.
The Katsumura patent shows a number of L.E.D.'s glued to an electrically insulating board, junction exposed and arranged to define a display. One diode is used per segment and no shroud is shown.
The Usui patent shows a metal layer with windows which serve as display segments. The patent also shows a number of diodes larger than the number of windows and not in registry with the windows. It appears that the junction of each diode is positioned with respect to the structure so that light is transmitted to an associated window for defining a display segment. Although the patent shows more than one diode associated with a window, there is no direct registry between the window and the diodes and the window is the size of a diode.
The Groves patent shows a plurality of L.E.D. arrays potted in a single cavity. No shroud is shown or discussed.